Filament coating boosts bulb efficiency

13 Jan 2016

Redirecting waste heat breathes new life into old bulbs, with further benefits expected for thermo-photovoltaics.

Conventional incandescent light bulbs have been outlawed in many countries mainly due to their inefficiency but also because they tend to fail after a relatively short lifetime. Typically, more than 95% of the electrical energy going into a tungsten-filamented bulb is wasted, mostly as heat.

But new research at MIT and Purdue University in the USA shows that there is a way to improve the efficiency of incandescent bulbs, the design of which has not changed much since development by prolific inventor Thomas Edison in 1879 (although his early filament material was carbonized bamboo).

The researchers have already achieved a bulb conversion efficiency of 6.6% but believe this could be increased to 40% with further refinement. Even the preliminary result matches the efficiency of some of today’s CFLs and LEDs; 6.6% represents a threefold improvement over today’s best conventional incandescent sources.

The team’s work was recently published in Nature Nanotechnology by three MIT professors — Marin Soljačić, professor of physics; John Joannopoulos, the Francis Wright Davis Professor of physics; and Gang Chen, the Carl Richard Soderberg Professor in Power Engineering — as well as MIT principal research scientist Ivan Celanovic, postdoc Ognjen Ilic, and Purdue physics professor (and MIT alumnus) Peter Bermel PhD (2007).

Ilic explained the overall aim of the research, which was not initiated with the aim of re-establishing the incandescent bulb industry. ”Most importantly,” he told optics.org, “this work wasn't about competing with LEDs, which we think are a great, ever-improving technology, that people should be using, but about the fundamental physics of tailoring thermal emission from a hot object. Our project is all about understanding the science of how and to what extent thermal emission from high temperature sources can be tailored through this concept of light recycling.”

How efficiency is raised

The key is to create a two-stage process, the researchers report. The first stage involves a conventional heated metal filament, with all its attendant losses. But instead of allowing the waste heat to dissipate in the form of infrared radiation, secondary structures surrounding the filament capture this radiation and reflect it back to the filament to be re-absorbed and re-emitted as visible light. The Nature Nanotechnology paper adds that these structures, a form of photonic crystal, are made of Earth-abundant elements and can be made using conventional material-deposition technology.

That second step makes a dramatic difference in how efficiently the system converts electricity into light. One quantity that characterizes a lighting source is the so-called luminous efficiency, which takes into account the response of the human eye. Whereas the luminous efficiency of conventional incandescent lights is between 2-3%, that of fluorescents, including CFLs, is between 7-15%, and that of most compact LEDs between 5-15%, the new two-stage incandescents could reach efficiencies as high as 40%, the team says.

The team refers to their approach as “light recycling,” said Ilic, since the coating material takes in the unwanted, useless wavelengths of energy and converts them into the visible light wavelengths that are desired. “It recycles the energy that would otherwise be wasted.”

Bulbs and beyond

One key to their success was designing a photonic crystal that works for a very wide range of wavelengths and angles. The photonic crystal itself is made as a stack of thin layers, deposited on a substrate.

Professor Marin Soljačić explained, “When you put together layers, with the right thicknesses and sequence you can get very efficient tuning of how the material interacts with light. In our system, the desired visible wavelengths pass right through the material and on out of the bulb, but the infrared wavelengths get reflected as if from a mirror. They then travel back to the filament, adding more heat that then gets converted to more light. Since only the visible ever gets out, the heat just keeps bouncing back in toward the filament until it finally ends up as visible light."

The technology involved has potential for many other applications besides light bulbs, said Soljačić. "The same approach could have dramatic implications for the performance of energy-conversion schemes such as thermo-photovoltaics. In a thermo-photovoltaic device, heat from an external source, whether chemical or solar or from a filament, makes a material glow, causing it to emit light that is converted into electricity by a photovoltaic absorber."

• The work was supported by the Army Research Office through the MIT Institute for Soldier Nanotechnologies, and the S3TEC Energy Frontier Research Center funded by the U.S. Department of Energy.

About the Author

Matthew Peach is a contributing editor to optics.org.